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The joint effects of competition and simulated grazing on an exotic grass and a co-occurring native species.
Gabbard, Bethany*,1, Fowler, Norma1, 1 University of Texas-Austin, Austin, TX, USA
ABSTRACT- Bothriochloa ischaemum (King Ranch bluestem grass) is a Eurasian species that has become common throughout central Texas. To investigate its effects upon the related native dominant, Schizachyrium scoparium (little bluestem grass), the separate and joint effects of competition and simulated grazing upon both species were measured in a fully factorial garden experiment. Plants were grown in pots for 5 months. Half of the 360 pots had B. ischaemum as the target species; the other half had S. scoparium as the target species. Each pot received one of three competition treatments (target plant grown alone; target plant grown with a conspecific plant; target plant grown with a plant of the other species). Each of the pots also received one of three clipping treatments (unclipped; clipped to 5 cm once; clipped to 5 cm twice). The final above-ground dry biomass of each species was greatly reduced by the presence of a B. ischaemum plant in the pot: B. ischaemum has a very strong competitive effect. In contrast, there was no significant effect of the presence in the pot of a S. scoparium plant on either species: S. scoparium appears to have a relatively weak competitive effect. The competitive response of S. scoparium was greater than the competitive response of B. ischaemum. Thus B. ischaemum outcompeted S. scoparium. Unfortunately, central Texas grasslands have few other species that are more likely than S. scoparium to outcompete B. ischaemum. Clipping had a significantly negative effect on both grass species; as expected, the negative effect of clipping was greater on S. scoparium, which is taller. We expected that, in the absence of clipping, the taller S. scoparium would outcompete the shorter B. ischaemum, but there was no interaction between the effects of clipping and competition: cessation of grazing is not likely to halt B. ischaemum.
Key words: competition, Bothriochloa ischaemum, grazing, invasive species